The present invention relates to an electromagnetic fuel injector provided with a plurality of coils for driving a valve body and to a technique for driving the valve body.
An electromagnetic fuel injector has a structure to control the opening and closing of a fuel passage between a valve body and a valve seat by attracting a plunger with the electromagnetic (attraction) force generated by passing current through a coil inside the injector and selectively separating the valve body from the valve seat, thereby to inject fuel through a fuel injection orifice. Such an injector is provided with a return spring as means for urging the valve body in a direction to press it against the valve seat as the magnetic attracting force for the plunger attenuates when current passage to the coil is interrupted, so that the fuel passage between the valve body and the valve seat is closed, that is, the valve is closed.
JP A 8-326620 discloses an injector in which two coils, a coil A and a coil B are provided, electric current is passed through both the coils A and B for a certain period from the start of current flow during the valve opening operation, the current flowing into the coil A is stopped at the end of the certain period, and current is then passed through only the coil B.
In the injector disclosed in JP A 8-326620, the characteristics of the two coils, coil A and coil B are not sufficiently considered. When it is desired to secure a high speed response of the valve operation, it is difficult to obtain the magnetomotive force necessary for maintaining the valve open in an condition, and when it is intended to secure a good stability during the time of holding the valve open, there is the possibility that obtaining a high response speed of valve operation is not attainable, that is, a condition is reached where an improvement in the response of the attracting force reaches a limit.
The inventors have studied an electromagnetic fuel injector which has a first coil (control coil) with a large time change ratio of magnetomotive force, which is a product of the number of turns of the coil and the magnitude of the current passing through the coil, and a second coil (hold coil) with a smaller time change ratio of magnetomotive force than the first coil, wherein a battery voltage is applied so that magnetic flux is generated in both of the control coil and the hold coil with the same orientation at an initial stage of the valve opening operation. This results in application of a lot of magnetomotive force to open the valve. However, only the magnetomotive force necessary to hold the valve open is generated by applying the battery voltage to only the hold coil after the valve has opened, whereby it is possible to achieve a high injection ratio and wide dynamic range without increasing the system cost.
According to a demand for a higher injection ratio and a greater degree of atomization of the fuel spray, the operating fuel pressure of an injector has tended to become higher. That is, the force necessary for holding the valve body in an open position also increases. Under such a condition, it is necessary to pass a large current through the hold coil or to increase the magnetomotive force by increasing the number of turns of the coil. However, it is not desirable to cause a large current to flow because of the accompanying increase in heat generation. Particularly, when the injector is driven with a low voltage, such as a battery voltage, a large current is necessary. The quantity of heat generation (power) is given by a product of the battery voltage and coil-flowing current. For example, when the current is increased to be twice, the heat generation amount becomes twice and the temperature elevation also becomes twice. Further, when the number of turns of the hold coil is increased, the inductance increases at the second power of the number of turns. For example, when the number of turns is increased to twice, the inductance become four times. When the inductance increases, a time delay occurs at rise, fall of the current, whereby a high speed driving of the valve body, which enables extension of the dynamic range, becomes impossible.
Here, an object of the present invention is to make it possible to generate a driving force characteristic which is most desirable for operation of an injector.
In order to attain the above object, in accordance with the present invention, an electromagnetic fuel injector is provided with a first coil having a large time rate of change of magnetomotive force and a second coil having a smaller time rate of change of magnetomotive force than the first coil. During the operation, at the time of opening the valve, current is passed through at least the first coil, and then current having a smaller current value than in the opening operation is caused to flow so as to hold the valve open, using the first and second coils.
In this specification, the opening operation at the time of valve opening refers to the valve operation in which the valve body is opened fully from the condition where the valve body is in contact with the valve seat, and it is distinguished over the valve open holding condition after that, in which the valve is held open. Further, in some cases, the opening operation is emphatically referred to as an initial stage of the opening operation.